UNITED NATIONS, April 2 (Reuters) - The United Nations said on Wednesday it has received 13 letters from Palestinian officials for accession to international conventions and treaties, adding it will review them to ensure the legal requirements of each instrument are met.

The 13 letters were delivered to Robert Serry, the U.N.'s Middle East envoy, as well as to the office of U.N. Secretary-General Ban Ki-moon in New York.

In a surprise decision, Palestinian President Mahmoud Abbas signed the letters on Tuesday, citing anger at Israel's delay of a prisoner release in a decision that jeopardized U.S. efforts to salvage fragile peace talks.

Their accession to those 13 treaties and conventions will now come into force within 30 days of their submission to the United Nations, Palestinian U.N. Ambassador Riyad Mansour told reporters on Wednesday.

The unexpected move was aimed at solidifying the standing of Palestinians in global bodies, defying both Israel and the United States that have long opposed such unilateral action.

The Palestinians were eligible to sign on to the treaties and conventions after the U.N. General Assembly upgraded the Palestinians' status at the United Nations in 2012 from "observer entity" to "non-member state," a move widely seen as de facto recognition of an independent Palestinian state.

"We are exercising our legal right as a state to join these instruments, we're part of humanity and this will allow us to shoulder our responsibility in carrying what is required from us under the provisions of these instruments," Mansour said.

Another two letters acceding to the Four Geneva Conventions and the Hague Convention respecting the Laws and Customs of War on Land were submitted to the Swiss and Dutch envoys to the Palestinian Authority, the United Nations said.

The Palestinian accession to the Geneva Conventions would take effect immediately because its territory was under occupation, Mansour said.

The Palestinians hope that by signing the Geneva Conventions, they will be given a stronger basis to eventually join the International Criminal Court and lodge complaints against Israel for its continued occupation of lands seized in the 1967 war.

"We have the right to do more and our leadership is acting in a very responsible way to do what is needed on the basis of the national interest of the Palestinian people," Mansour said.

Abbas had pledged not to seek to join world bodies during the U.S.-brokered negotiations, which are scheduled to run until the end of April and have made little apparent headway so far.

Israel had promised in exchange to free more than 100 prisoners by the end of March, but failed to release the final batch, saying it wanted guarantees that the Palestinians would extend the negotiations beyond the April 29 deadline.

The United Nations said the 13 letters of accession it had received from the Palestinian Authority related to:

* The Vienna Convention on Diplomatic Relations

* The Vienna Convention on Consular Relations

* The Convention on the Rights of the Child and the Optional Protocol to the Convention on the Rights of the Child on the Involvement of Children in armed conflict

* The Convention on the Elimination of All Forms of Discrimination against Women

* The Convention on the Rights of Persons with Disabilities

* The Vienna Convention on the Law of Treaties

* The International Convention on the Elimination of All Forms of Racial Discrimination

* The Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment

* The United Nations Convention against Corruption

* The Convention on the Prevention and Punishment of the Crime of Genocide

* The International Convention on the Suppression and Punishment of the Crime of Apartheid

* The International Covenant on Civil and Political Rights, and

* The International Covenant on Economic, Social and Cultural Rights. (Reporting by Michelle Nichols, editing by G Crosse)